Acoustic baffle for deep submergence

ABSTRACT

A multi-layered acoustic baffle, particularly useful at great depths, comprising at least two layers of fume-produced silicon dioxide, the particle size being between 70 to 500 angstroms, the particles being sintered together in chain-like formations and having surface areas of 50 m2/g to 400 m2/g; each of the layers being prestressed to a pressure greater than the pressure at which the baffle is to be used; and each of the layers having a thickness not greater than 1/4 wavelength of the highest frequency of interest in the material. At least one layer of high-density, viscoelastic, material having a high acoustic shear loss coefficient is sandwiched in between each layer of the silicon dioxide. The baffle further comprises two layers of a compliant watertight coating bordering the two outer layers of silicon dioxide and the sides of the baffle, thereby completely enclosing the other layers.

Unite it? Johansen States Patent 1191 1 Dec.2,1975

[ ACOUSTIC BAFFLE FOR DEEP SUBMERGENCE [75] Inventor: Carl R. Johansen, La Verne, Calif.

[73] Assignee: The United States of America as represented by the Secretary of the Navy, Washington, DC.

[2 2] I Filed: June 19, 1972 21 Appl.- No.: 263,893

[52] U.S. Cl. 181/.5; 181/33 G; 181/51; 181/180; 343/18A [51] Int. Cl. C09K 3/00 58 Field of Search 343/18 A, 18 R; 181/.5 R, 181/33 G, 33 GR, 5'1, 180

Primary Examiner-Malcolm F. Hubler Assistant ExaminerN. Moskowitz Attorney, Agent, or Firm-Richard S. Sciascia; Ervin F. Johnston; John Stan [57] ABSTRACT A multi-layered acoustic baffle, particularly useful at great depths, comprising at least two layers of fumeproduced silicon dioxide, the particle size being between 70 to 500 angstroms, the particles being sintered together in chain-like formations and having surface areas of 50 m /g to 400 m lg; each of the layers being prestressed to a pressure greater than the pressure at which the baffle is to be used; and each of the layers having a thickness not greater than 1/4 wavelength of the highest frequency of interest in the material. At least one layer of high-density, viscoelastic, material having a high acoustic shear loss coefficient is sandwiched in between each layer of the silicon dioxide. The baffle further comprises two layers of a compliant watertight coating bordering the two outer layers of silicon dioxide and the sides of the baffle, thereby completely enclosing the other layers.

6 Claims, 3 Drawing Figures 1E 14' 52/445- V/s'ea Eusm: mwesa M4 1521.41, flame-555p 16 CoMpuAA/f Mrs/oval" C04 TIA/G.

14A! Acausnc EAFFLiQ/Q DEEP St/EMEkGEA/CE APPLICAf/OAS.

ACOUSTIC BAFFLE FOR DEEP SUBMERGENCE STATEMENT OF GOVERNMENT INTEREST The invention described herein may be manufactured and used by or for the Government of the United States of America of governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION There are two operationally separate but not exclusive divisions which the function'of the baffle of this invention might be divided into: underwater sound baffling, and underwater anechoic paneling, that is, paneling having an unusually low amount of reverberation. Underwater baffles limit incident sound transmitted through them by a combination of reflection and absorption, but may reflect any amount of sound. Anechoic materials need not prevent sound transmission, but must limit sound reflection by a combination of transmission and absorption. Following are the principle sound baffling methods in use today:

1. I-Iollow, empty, compliant tubes formed into an appropriate geometrical arrangement. These structures are acoustic reflectors which are rather complex, expensive, and only effective over a limited frequency range.

2. Air-filled rubber systems, that is, foam rubbers. These are pressure-(depth) and frequency-dependent reflectors.

3. Corprene layers formed into the appropriate shape. Corprene is a widely used word to designate a mixture of neoprene and ground up cork. More specifically, corprene refers to cork particles in a rubber matrix. This material is an effective reflector at shallow depths (low pressure), but the pressure associated with increased depth causes serious degradation in its performance.

4. Multicelled gas pressurized systems. These systems form effective reflecting baffles; however, the high pressure gas in the system presents severe safety problems in the submarine environment.

5. Conical disk spring baffle plate as described in Pat. No. 3,277,434. This system is complex to construct, expensive, and limited in its effectiveness, except at a specified operating pressure.

Satisfactory operational pressure-insensitive anechoic materials do not exist at this time: The present methods include rubber void systems designed to be anechoic. These materials are quite pressure-sensitive and operate over a narrow frequency range.

SUMMARY OF THE INVENTION This invention relates to a multi-layered acoustic baffle, particularly useful at great depths underwater, comprising at least two layers of fumeproduced silicon ide and the sides of the baffle completely enclose the layers.

,OBJECTS OF THE INVENTION An objectof the invention is to provide a baffle which serves two functions: as underwater sound baf- BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a cross-sectional view of the acoustic baffle I of this invention, for deep submergence applications.

FIG. 2 is a greatly enlarged representation of submicron silicon dioxide agglomerates.

FIG. 3. comprises a pair of graphs showing the variation of density and velocity with variation of a specific viscoelastic material, Silastic".

dioxide, the particle size being between 70 to 500 angstroms, the particles being sintered together in chainlike formations and having surface areas of 50 m /g to 400 mlg. Each of the layers is prestressed to a'pressure greater than the pressure at which the baffle is to be used, and each of the layers has a thickness not greater then A wavelength of the highest frequency of interest in the material. At least one layer of high-density, viscoelastic, material, having a high acoustic loss coefficient shear, is sandwiched in betweeneachlayer of the silicon dioxide. Two layers of a compliant watertight coating bordering the two outer layers of silicon diox- DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the figures, FIG. 1 shows one embodiment 10 of the invention. Material 12 is composed of fume-produced silicon dioxide particles such as CAB-O-SIL, which have been pre-processed in forming with a pressure in excess of the required operating pressures. CAB-O-SIL is a trademark for a fumed silica manufactured by the Cabot Corp., High St., Boston, Mass, 02110.

This preprocessing is discussed in great detail in US. Pat. No. 3,542,723, to S. F. Sullivan et al, which issued on'Nov. 24, 1970. In brief, the finely divided particles of fumed silica, which may be mixed with a phenolic binder, are compressed in a mold, or other type of confining walls, to a pressure, for example of as high as 3000 psi if the material is to be used subsequently at an operating stress or pressure somewhat less than 3000 psi. The structural element, which now has the desired shape, retains the characteristics of a linear elastic pressure release material having a substantially linear acoustic relative impedance when thereafter subjected to the operating stress.

The morphology of the basic silicon dioxide or glass particles is of primary importance. Individual spheroidal particles, which may be of a size of 0.050 micron or less, fuseto one another in the manufacturing process to form a very irregular, low-density, flexible, submicroscopic agglomerate, as is indicated in FIG. 2. The compacted agglomerates form a uniform bulk material with a wide range of acoustic characteristics. See the table.

Besides the particles of fumed silicon-dioxide, the acoustic baffle 10 may further comprise a phenolic binder within which the finely divided silica particles are mixed, the mixture of the silica particles and the phenolic binder being first prestressed into a desired form.

Instead of a phenol, the material termed Silastic may be used in conjunction with the silicon dioxide, to

form a matrix of silicon dioxide and Silastic. Silastic a trademark of the Dow Corning Corp. of Midland, Mich., and relates to compositions of milled and compounded rubber prior to vulcanization, but containing organosilicon polymers. The wide variation of the parmeters density and velocity with changes in the percentage of Silastic may be seen in H0. 3. Since the modulus and the characteristic impedance are also functions of the density and velocity, they also may be determined from the chart.

The layers 14 are composed of high-density viscoelastic material which has a high acoustic loss coefficient in shear, for example 0.3, e.g. lead-loaded vinyl, which is commercially available in sheets, or a silicon rubber compound, also available commercially. While these materials are also available with a low acoustic loss coefficient in shear, low-loss materials are not of particular interest in this invention.

The desired thickness of the various layers, particu larly layer 12, may be readily calculated, using standard transmission line theory for acoustic frequencies. A very useful reference is Physical Acoustics, Volume I, Part A, edited by Warren P. Mason, and published by the Academic Press. Particularly pertinent is Chapter 4, Section II of this reference.-

The thickness of the various layers would be functions of the desired impedance and propagation constants. If it be desired to have a flat frequency response at low frequencies, the layers of silicon dioxide 12 should be no thicker than a quarter wavelength for the highest acoustic frequency of interest in the material.

The thicknesses of layers 14 and 16 are in the range of to 100 mils.

Layer 16 comprises a compliant watertight coating. Its function is to keep the interior of the baffle 10 dry without stiffening the structure 10. Neoprene would serve this function. The thickness and number of sections of layers 12 and 14 can vary according to economic and funcitonal tradeoffs. There must be at least one layer 12. With only one layer 12, the system will behave in a normally predictable manner for an acoustic transmission line having the acoustic parameters of the bulk material 12. Thickness is one of these parameters.

The sandwich construction of the baffle 10 has the additional merit of (1) eliminating the large shear wave component in the layers of silicon dioxide 12. Too large a shear wave component in layer 12 will result in an acoustic impedance mismatch between the layer 12 and the ambient seawater. There must be anechoic characteristics in the system, but introducing these characteristics results in a phase shift and causes the reflection coefficient to decrease because of phase problems; (2) Another advantage of the sandwich construction of the baffle 10 is that of introducing acoustic absorption into the system. The reflection coefficient is decreased as the series acoustic impedance of the system approaches that of water.

The structural nature of material 12 causes a threedimensional, frequency-independent, equipartition of energy in a transmission distance of less than a millimeter.

The equipartition of energy is determined by the mean free path of the mechanical displacement, and in the material of the silicon dioxide the structure is so fine that the mean free path is of the order of a linear array, or chain, of several silicon dioxide particles, shown by reference numeral 22 in FIG. 2. An equiparti- 4 tion of energy is obtained in several mean free path lengths, as far as mechanical energy is concerned. This would genrally be less than 1 mm.

Associated with this equipartition of energy, there is an equal particle displacement in three orthogonal directions in the material of the silicon dioxide 12, even when the system is subjected to high pressures. Interaction between the material 12 and the material 14 at the interface of the two results in a high acoustic absorption. There is an energy loss of up to two thirds of the available energy at each interface of the two where the energy associated with directions parallel to the interface is absorbed by means of a shear conversion across the interface. The acoustic parameters, including thickness, of the viscoelastic material 14 should be optimized using ordinary acoustic transmission line theory to provide the greatest energy density at each interface consistent with maintaining an acceptable system impedance match with the outside environment.

Items (1) and (2) hereinabove, with respect to the merits of the sandwich type construction, relate to the reflection coefficient of the material. The sandwich construction allows control of, in a basically independent manner, parameters that contribute to the reflection coefficient. There is obtained the additional degree of freedom that is needed to produce an anechoic material, or to produce a lossy material and at the same time have impedance matching characteristics so as to have a low reflection coefficient associated with the anechoic material.

The system herein described is simple and consequently potentially more reliable and cheaper than existing systems. As a consequence of the combination of materials as described, the system will act as an effective, pressure-insensitive and frequency-insensitive, baffle and/or anechoic absorber at the high pressures associated with operational depths in the ocean, up to several thousand feet. The specific characteristics of the baffle-absorber system can be specified and controlled over wide ranges during manufacture by designing the system using the methods indicated and described hereinabove. Some of these parameters are the acoustic velocity, density, and acoustic impedance. The range of their variations are shown in the table. The combination of materials in the manner described and the understanding and use of the interface absorption phenomena in this type of system is believed to be novel.

As stated hereinabove, the morphology of the material is of primary importance. The density, velocity, and other characteristics depend on the overall compliance and porosity of the material, while the equipartition of energy characteristic depends on the random nature of the microscopic particles. Pure submicron silicon dioxide particles of this type have proven to be rather difficult to handle in practice. Tests have shown that a small percentage of any material which is compliant compared to bulk silicon dioxide, or glass, which can be uniformly dispersed among the submicron silicon dioxide particles can be used as a stablizer to improve the handleability of the finished product. Examples of this type of compliant material are silicone rubber and most, if not all, phenols, as well as Silastic. There is no deterioration of the desired properties of the final material provided care is used during the mixing process not to change the basic structure of the submicron silicon dioxide agglomerates.

An alternate method of making an anechoic absorber is to uniformly distribute the viscoelastic material 14 throughout the volume of material 12 in such a manner that the basic submicron silicon dioxide agglomerates are not changed. In principle this is the same system as that originally described, however, the individual layers are each taken to the limit of zero thickness while adding additional layers in such a way as to keep the total thickness of the baffle constant. This process will greatly increase the interface area between materials 12 and 14, resulting in a higher absorption coefficient. From a manufacturing viewpoint and on a macroscopic level, the combination of layers 12 and 14 could now be considered to be a uniform material.

TABLE Ranges of Acoustic Parameters Achievable with Compacted Submicron Silicon-Dioxide Agglomerates.

ACOUSTIC VELOCITY 600 m/sec to 2500 m/sec DENSITY 0.5 gm/cm to 2 grn/cm ACOUSTIC IMPEDANCE each of the layers being prestressed to a pressure greater than the pressure at which the baffle is to be used; and

each of the layers having a thickness not greater than A wavelength of the highest frequency of interest in the material;

at least one layer of high-density, viscoelastic material having a high acoustic shear loss coefficient, sandwiched in between each layer of the silicon dioxide; and

a layer of a compliant watertight coating bordering the two outer layers of silicon dioxide and the sides of the baffle, thereby completely enclosing the other layers.

2. The acoustic baffle according to claim 1, wherein the high-density viscoelastic material comprises a composition, prior to vulcanization, of milled and compounded rubber containing organosilicon polymers.

3. The acoustic baffle according to claim 2, wherein the compliant watertight coating comprises neoprene.

4. The acoustic baffle according to claim 3, wherein the layers of silicon dioxide and of viscoelastic material are very thin, with respect to the highest frequency of interest, while maintaining the total thickness of the baffle constant, so that in the limit the layers approach zero thickness and the baffle may now be considered to be made of a uniform material.

5. The acoustic baffle according to claim 3, further comprising:

a phenolic binder within which the finely divided silica particles are mixed, the mixture of the silica particles and the phenolic binder being prestressed into a desired form.

6. The acoustic baffle according to claim 4, further comprising:

a phenolic binder within which the finely divided silica particles are mixed, the mixture of the silica particles and the phenolic binder being prestressed into a desired form. 

1. A MULTI-LAYERED ACOUSTIC BAFFLE, PARTICULARLY USEFUL AT GREAT DEPTHS, COMPRISING: AT LEAST TWO LAYERS OF FUME-PRODUCED SILICON DIOXIDE, THE PARTICLE SIZE BEING BETWEEN 70 TO 500 ANGSTROMS, THE PARTICLES BEING SINTERED TOGETHER IN CHAIN-LIKE FORMATIONS AND HAVING SURFACE AREAS OF 50 M2/G TO 400 M2/G; EACH OF THE LAYERS BEING PRESTRESSED TO A PRESSURE GREATER THAN THE PRESSURE AT WHICH THE BAFFLE IS TO BE USED; AND EACH OF THE LAYERS HAVING A THICKNESS NOT GREATER THAN 1/4 WAVELENGTH OF THE HIGHEST FREQUENCY OF INTEREST IN THE MATERIAL; AT LEAST ONE LAYER OF HIGH-DENSITY, VISCOELASTIC MATERIAL HAVING A HIGH ACOUSTIC SHEAR LOSS COEFFICIENT, SANDWICHED IN BETWEEN EACH LAYER OF THE SILICON DIOXIDE; AND A LAYER OF A COMPLIANT WATERTIGHT COATING BORDERING THE TWO OUTER LAYERS OF SILICON DIOXIDE AND THE SIDES OF THE BAFFLE, THEREBY COMPLETELY ENCLOSING THE OTHER LAYERS.
 2. The acoustic baffle according to claim 1, wherein the high-density viscoelastic material comprises a composition, prior to vulcanization, of milled and compounded rubber containing organosilicon polymers.
 3. The acoustic baffle according to claim 2, wherein the compliant watertight coating comprises neoprene.
 4. The acoustic baffle according to claim 3, wherein the layers of silicon dioxide and of viscoelastic material are very thin, with respect to the highest frequency of interest, while maintaining the total thickness of the baffle constant, so that in the limit the layers approach zero thickness and the baffle may now be considered to be made of a uniform material.
 5. The acoustic baffle according to claim 3, further comprising: a phenolic binder within which the finely divided silica particles are mixed, the mixture of the silica particles and the phenolic binder being prestressed into a desired form.
 6. The acoustic baffle according to claim 4, further comprising: a phenolic binder within which the finely divided silica particles are mixed, the mixture of the silica particles and the phenolic binder being prestressed into a desired form. 